Deep Exclusive π " Production with transversely polarized He3 using SoLID

Transcription

1 Deep Exclusive π " Production with transversely polarized He3 using SoLID A run-group proposal with E Zhihong Ye, ANL On behalf of Co-Spokespeople: Garth Huber (contact), Zafar Ahmed, from Univ. of Regina and Zhihong Ye 05/07/2016 Drafted proposal:

2 Physics Motivation Generalized Parton Distribution: Ø GPDs give the 3D spatial distributions of quarks and gluons in a nucleon Ø GPDs interrelate the longitudinal and transverse momentum structure of partons within a fast moving hadron. Ø At leading twist-2, four quark chirality conserving GPDs for each quark, gluon type. Ø Because quark helicity is conserved in the hard scattering regime, the produced meson acts as a helicity filter. H (,* x, ξ, t spin avg. no heli. flip H% (,* x, ξ, t spin diff. no heli. flip E (,* x, ξ, t spin avg. helicity flip E' (,* x, ξ, t spin diff. helicity flip Ø Leading order QCD predicts: Vector meson production sensitive to unpolarized GPDs, H and E. Pseudoscalar mesons sensitive to polarized GPDs, H% and E'. 2

3 Physics Motivation Generalized Parton Distribution: Ø Integral of transverse components reduces GPDs into one-dimensional PDF Ø Access to Angular Momenta of quarks & gluons. Ø First moments of GPDs are related to nucleon elastic form factors through modelindependent sum rules: 45. e ( 0 dxh ( (x, ξ, t) ( "5 45. e ( 0 dxe ( (x, ξ, t) ( "5 = F 5 (t) = F 8 (t) } t Dirac and Pauli elastic nucleon form factors. -dependence fairly well known. 45. e ( 0 dxh% ( (x, ξ, t) ( "5 = G : (t) Isovector axial form factor. t dep. poorly known. 45. e ( 0 dxe' ( (x, ξ, t) ( "5 = G ; (t) Pseudoscalar form factor. Very poorly known. 3

4 Physics Motivation Factorization of Hard Reactions: Ø Hard probe creates a small size qq= or gluon configuration (or a real photon in DVCS); (described by pqcd) { Ø Non-perturbative part describes how hadron reacts to this configuration, or how the probe is transformed into hadrons; (parameterized by GPDs). Factorization { L ü ü Hard exclusive meson electroproduction first shown to be factorizable by Collins, Frankfurt & Strikman [PRD 56(1997)2982]. Factorization applies when the γ* is longitudinally polarized. ü corresponds to small size configuration compared to transversely polarized γ*. 4

5 Physics Motivation Exclusive Hard Processes to probe GPDs: Ø Deeply Virtual Compton Scattering (DVCS): ü Sensitive to all four GPDs. Ø Deep Exclusive Meson Production (DEMP): ü Vector mesons sensitive to spin-average H, E. ü Pseudoscalar mesons sensitive to spin-difference, H% and E'. Ø Need a variety of Hard Exclusive Measurements to disentangle different GPDs. 5

6 Physics Motivation Probe GPD-E' with DEMP:. e ( 0 dxe' ( (x, ξ, t) ü GPD-E' is not related to an already known parton distribution. ( 45 "5 = G ; (t) ü Experimental information can provide new nucleon structure info unlikely to be available from any other source. ü G P (t), which is highly uncertain, receives contributions from J PG =0 -- states, and contains an important pion pole contribution. Pion pole contribution to G P (t) Pion pole contribution to meson electroproduction at low t. For this reason, a pion pole-dominated ansatz is typically assumed: where F π is the pion FF and φ? the pion PDF. 6

7 Physics Motivation Target Single Spin Asymmetry in DEMP: Ø Asymmetry with transversely polarized target and longitudinally polarized virtual phone Ø Unpolarized Cross section dσ π L = exclusive π cross section for longitudinal γ* β = angle between transversely polarized target vector and the reaction plane. Ø Polarized cross section has additional components sin β module L/T Separation A = y 1 2 2σL P π σ L 7

8 Physics Motivation Target Single Spin Asymmetry in DEMP: Ø Frankfurt et al. have shown A L vanishes if E' is zero [PRD 60(1999)014010]. n If E' 0, the asymmetry will display a sinβ dependence. n Higher order corrections, which may be significant at low Q 2 for σ L, likely cancel in A L. Ø Belitsky and Müller calculations: ü At Q 2 =10 GeV 2, NLO effects can be large, but cancel in A L (PL B513(2001)349). ü At Q 2 =4 GeV 2, higher twist effects even larger in σ L, but still cancel in the asymmetry (CIPANP 2003). This relatively low value of Q 2 for the expected onset of precocious scaling is important, because it is experimentally accessible at JLab 12 GeV. 8

9 Physics Motivation Target Single Spin Asymmetry in DEMP: Full calculation, including transverse and longitudinal γ HERMES Data: Ø Exclusive π + production by scattering 27.6 GeV positrons or electrons from transverse polarized 1 H without L/T separation. [PLB 682(2010)345] Ø x B =0.13, Q 2 =2.38 GeV 2, -t =0.46 GeV 2. Longitudinal γ only ü Goloskokov and Kroll indicate the HERMES results have significant contributions from transverse photons, as well as from L and T interferences. [Eur Phys.J. C65(2010)137] ü Because no factorization theorems exist for exclusive π production by transverse photons, these data cannot be simply interpreted in terms of GPDs. 9

10 Physics Motivation Target Single Spin Asymmetry in DEMP: Ø The study of A L is also important for the reliable extraction of F π from p(e,e π + )n data at high Q 2. [Frankfurt, Polyakov, Strikman, Vanderhaeghen PRL 84(2000)2589]. Non-pion pole contributions need to be accounted for in order to reliably extract F π from σ L data at low -t. 12 GeV Pion Form Factor experiment restricted to Q 2 =6 GeV 2 to keep non-pole contributions to an acceptable level (-t min <0.2 GeV 2 ). Ø A L is an interference between pseudoscalar and pseudovector contributions. Help constrain the non-pole contribution to p(e,e π + )n. Assist the more reliable extraction of the pion form factor. Possibly extend the kinematic region for F π measurements. Ø To cleanly extract A L, we need: Target polarized transverse to γ* direction. Large acceptance in π azimuthal angle (i.e. φ, β). Measurements at multiple beam energies and electron scattering angles. ε dependence (L/T separation); controlled systematic uncertainties 10

11 Physics Motivation Complementarity of SoLID and SHMS+HMS Experiments SHMS+HMS: SoLID: HMS detects scattered e. SHMS detects forward, high momentum π. Expected small systematic uncertainties to give reliable L/T separations. Good missing mass resolution to isolate exclusive final state. Multiple SHMS angle settings to obtain complete azimuthal coverage up to 4 o from q-vector. It is not possible to have complete azimuthal coverage at larger t, where A L is largest. PR by GH, D. Dutta, D. Gaskell, W. Hersman. n n n n n Complete azimuthal coverage, polar angle θ= 8 o up to 24 o for e and π High luminosity, particle ID and vertex resolution capabilities well matched to the experiment. L/T separation is not possible, the asymmetry is diluted by T, TT contributions. The measurement is valuable as it is the only practical way to obtain A sin(φ-φ UT s ) over a wide kinematic range. Complementary to Hall C measurement. 11

12 Measure DEMP with SoLID-SIDIS n e, e A π " p:with transversely polarized He3 Run in parallel with E : E0 = 11.0 GeV (48 days) Luminosity = cm -2 s -1 (per nucleon) Coil Inner Diameter (m) Inner 1.27 Outer 1.45 Vertical 1.83 Note: The target size is roughly close to real but the location is estimated. 12

13 Measure DEMP with SoLID-SIDIS Large-Angle : Detect electrons and protons Forward-Angle : Detect electrons & pions & protons e/π ± Coverage: àforward Acceptance: φ: 2π, θ: 8 o o, P: GeV/c, for e/π ± àlarge Acceptance: φ : 2π, θ: 16 o -24 o, P: GeV/c, for e only Proton Coverage : àsame to e/pi at FA and LA, except the momentum-range can be much lower Resolution: δp/p ~ 2%, θ ~ 0.6mrad, φ ~ 5mrad Online Coincidence Trigger: Electron Trigger + Hadron Trigger Offline Analysis: Identify protons and form triple-coincidence 13

14 A Conceptual Design: ü Cover angles of 24 o to 50 o 2π on the azimuthal angle ü Inner Radius=32 cm Outer Radis = 67 cm Detector Length = 50 cm ü Distance from Target = 79cm (far end touches the magnet) Proton Recoil Detector Ø Need good timing resolution (<60~ps) Ø Need fine segments due to huge low energy backgrounds (An aluminum foil cover can block most of low energy electrons) Ø Need to provide angle information for offline background suppression Ø Photon-Detectors need to work in strong magnetic fields from target & solenoid Ø A good candidate: Scintillating Fiber Tracker Ø Geant4 Simulation is undergoing 14

15 Scintillating Fiber Tracker SiPM àavalanche Photodiode (APD) pixels working in Geiger-mode A 10cm2 SciFi-Tracker made by a medical group A prototype project funded by JSA 2014 Postdoc Prize EASIROC+SiPM ( portable!) 15

16 Proton Detection Using Time-Of-Flight as Proton-PID: Need >5σ timing resolution to identify protons from other charged particles. Ø Existing SoLID Timing Detectors: ü Timing + Momentum Info ü MRPC & FASPD at Forward-Angle cover 8 0 ~ ; >3ns separation ü LASPD at Large-Angle cover 14 0 ~ 24 0 ; >1ns separation Ø A new Proton Recoil Detector: No momentum info; Only reply on 1D TOF cuts ü Cover 14 0 ~ 24 0 ; ü Good separation from electrons/pions (lowp π " highp p >=0.3ns, need σ < 60ps) Hard to separate kaons from protons low-p K mixed with high-p p 16

17 Projection Acceptance: (Cuts on Q2>4 GeV 2 and W>2 GeV) 17

18 Kinematic & Rates: Projection Ø Rates were estimated with a model developed by Garth & Zafar. Ø Good physics rates are at Q 2 >4GeV 2 : 0.53 Hz (or 0.31 Hz w/o PRD) Ø Dominated background are SIDIS events 18

19 Asymmetry Binning: Ø 1D binning on t (May further binning on Q 2 ) Projection Ø Asymmetries are diluted due to not doing L/T separation: from model: Ø Stat. Error is given as: Pà He3 polarization 60% η \ à Effective neutron fà Dilution from protons (0.9 estimated) 19

20 Asymmetry Binning: Ø 1D binning on t (May further binning on Q 2 ) Projection Ø Asymmetries are diluted due to not doing L/T separation: from model: Ø Stat. Error is given as: Pà He3 polarization 60% η \ à Effective neutron fà Dilution from protons (0.9 estimated) 20

21 Exclusivity of DEMP Events Missing Mass Ø With Proton detection, most of background events can be suppressed Ø Major background would be SIDIS events from (a) Protons in X, (b) Accidental coincidence of SIDIS events with protons in all background sources Ø Reconstructing Missing Momentum and Missing Mass to further suppress background during offline analysis. ü Assuming all X in SIDIS contain protons (hard to estimate the real branching-ratio) ü Fold in detector resolutions: ]^ ^ = 8%`,δθ = 0.6 mrad, δφ = 5 mrad ü Nucleus-Effect, Fermi Motion and Radiative Effect are not considered yet but some of them are expected to be small in the asymmetry extraction. 21

22 Exclusivity of DEMP Events Missing Mass ü Missing Momentum are well separated for SIDIS and DEMP. ü Cutting P miss <1.2 GeV/c, reject most of SIDIS background ü Background is expected to be even smaller, since SIDIS rate are overestimated ü Other backgrounds will be more uniform in the MM, asymmetries of which can be evaluated and corrected. ü Rest of random background will largely suppressed in the asymmetry. SIDIS rate is still overestimated for safety 22

23 Systematic Uncertainties Detector-wide, DEMP measurement shares the same systematic uncertainties with SIDIS experiments: Other sources of uncertainties are still under estimation. 23

24 To-Do-List Projections were made by assuming a free neutron. We are implementing Fermi- Motion and Radiative Effect in the generator. Optimizing the projections, e.g. further binning on Q 2 Further designing and evaluating the Proton-Recoil-Detector Study more background situation Evaluating more systematic errors Double check of all calculations 24

25 Summary GPDs provide new information of the 3D spatial distributions of quarks and gluons; connect 1D- PDF, Form-Factors and so on. (Four GPDs for each quark flavor or gluon: H, E, H% and E') DEMP can measure H% and E'; It is an unique process to probe E', which gets access to pion form factors. Target Single-Spin Asymmetry of DEMP has relatively low requirement on the Q 2 (Higher order effects are largely cancelled even at Q 2 ~4 GeV 2 ). Using SoLID-SIDIS configuration and transversely polarized He3, we can measure asymmetries of neutron DEMP Complementary to the Hall-C experiment with limited coverage but doing L/T separation Run in-parallel with SIDIS experiments; No new beam time; No configuration change needed except potentially adding a new proton recoil detector Expect to have very good statistical errors over wide t coverage Proton Detection will help us to maintain the Exclusivity. Missing Momentum and Missing Mass cuts can further reject most background. 25

26 Backup Slides 26

27 DEMP TSSA Connection to GPDs L. Frankfurt et. al., PRD (1999): Charge Pion Production: 27

28 Acceptance w/ Q2>1GeV^2 Cut 28